The present invention relates to a core substrate having an electrically conductive core section, a method of producing the core substrate and a method of producing a circuit board having the core substrate, more precisely relates to a core substrate capable of solving a problem of expansion of cable layers on the substrate, which is caused by a gas generated from the substrate when a thermal treatment is performed, a method of producing the core substrate and a method of producing a circuit board having the core substrate.
Some test substrates, which are used for testing circuit boards, on which semiconductor elements will be mounted, and semiconductor wafers, include core substrates composed of carbon fiber-reinforced plastic (CFRP). In comparison with conventional glass-epoxy core substrates, thermal expansion coefficients of the core substrates composed of carbon fiber-reinforced plastic are small, and thermal expansion coefficients of the circuit boards having such core substrates can be corresponded to those of semiconductor elements to be mounted on the circuit boards. Therefore, thermal stress generated between a semiconductor element and a circuit board can be effectively avoided.
The circuit board is formed by laminating cable layers on the both side faces of the core substrate, and plated through-hole (PTH) sections are formed in the core substrate so as to mutually electrically connect the cable layers on the both side faces thereof. The plated through-hole sections are formed by boring through-holes in a substrate and forming plated layers (electrically conductive parts) on inner faces of the through-holes.
In case of the core substrate having the electrically conductive core section composed of, for example, carbon fiber-reinforced plastic, if the plated through-hole sections are formed by merely boring the through-holes and plating the inner faces thereof, the plated through-hole sections and the core section are electrically shorted. Thus, the plated through-hole sections are formed in the core substrate having the electrically conductive core section by the steps of: forming pilot holes, whose diameters are greater than those of the plated through-hole sections to be formed, in the core substrate; filling the pilot holes with insulating resin; and forming the plated through-hole sections in the filled through-holes. With this method, the plated through-hole sections and the core section are not electrically shorted (see JP Kohyo Gazette No. 2004/064467, JP Patent Gazette No. 2006-222216).
However, if the pilot holes are drilled, burrs are formed on inner faces of the pilot holes and the plated through-hole sections and the core section will be electrically shorted. To solve this problem, the inner faces of the pilot holes are coated with insulating layers so as not to electrically short the plated through-holes and the core section (see JP Patent Gazette No. 2006-222216). However, it is difficult to perfectly coat the rough inner faces of the pilot holes.
In a production process of a circuit board, prepregs and cable sheets are piled, heated and pressurized so as to integrally laminate them, so that the cable layers can be laminated. In this case, there are a disadvantage of generating a decomposition gas from an insulating material and vaporizing moisture absorbed in a substrate while performing the heating step. Thus, some means for discharging gasses from inner layers of the substrate have been employed (see JP Patent Gazettes No. 2004-87989 and No. 2005-136347).
In case of the core substrate having pilot holes for forming the plated through-hole sections, the gasses, which are generated in the substrate while producing the substrate, will cause the problem of short circuit between the core section and the plated through-hole sections. This short circuit must be prevented.